Travelling wave amplifier tubes



April 29, 196 9 N. w. HARRIS ETAL. 3,441,783

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April 29, 1969 N. w. HARRIS ET AL 3,441,783

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United States Patent Ofiice Int. Cl. H013 25/34, 19/80 US. Cl. 315-35 11 Claims ABSTRACT OF THE DISCLOSURE A travelling wave tube having a coupling device between its retarded wave structure and an associated output waveguide, the coupling device being of the coupling hole and tuning stub type having a shape which is not a circular shape symmetrical about the axis. Either the tuning stub or the coupling hole may be non-circular.

This invention relates to travelling wave amplifier tubes and more particularly to such tubes of the kind in which power is coupled out from the delay line or retarded wave structure of the tube to a waveguide or other transmission line or is coupled into said delay line or retarded wave structure from said waveguide or line by a coupling device incorporated in the structure of the tube at the end thereof.

The invention is illustrated in and further explained in connection with the accompanying drawings. FIG- URES 1, 2 and 3 of these drawings are provided for purposes of explanation and show a typical known travelling wave tube, FIGURE 1 being a schematic longitudinal view (shown broken away centrally), FIGURE 2 being an enlarged sectional view of an output coupling device at the output end of the tube, and FIGURE 3 being a simplified transverse sectional view on the line 3-3 of FIGURE 2. FIGURES 4 to 9 inclusive are views of the same nature as that of FIGURE 3 illustrating different embodiments of the present invention.

Referring to FIGURES 1 to 3 the known tube therein shown comprises a cathode gun assembly 10 carried by means including an insulator 11 which forms part of the tube envelope and which is sealed to the anode 12. The retarded wave structure is indicated at 14 which is indicated as consisting of a number of inductively coupled resonators. The retarded wave structure may be of any form known per se in travelling wave tubes: for example it may be of the known long slot type described by Allen and Kino in the Proceedings of the Institution of Radio Engineers Trans, vol. MTT-8, at pages 362 to 372, or of the loop coupled centipede type or of the cloverleaf type both of which are described by Chodorow and Craig in the proceedings of the same institution, in vol. 45 pp. 1106-1118. The illustrated tube shows the cloverleaf type.

The parts of the tube are aligned so that, in use and with a suitable high negative potential applied to the gun, it projects a narrow beam of electrons axially through the anode 12 and the retarded wave structure 14 to a fluid-cooled collector 18 which is at the end of the tube remote from the gun and is mounted on a plate 15. The plate is secured to the output end of the retarded wave structure and forms part of a coupling device by means of which output energy is fed to an output waveguide 16 terminated at its outer end by a vacuum-tight wavepermeable window 17. As is well known, when in use a high frequency wave travels down the retarded wave structure at substantially the same speed as the electrons of the beam within it, amplification being obtained by inter-action of the beam with said structure.

3,441,783 Patented Apr. 29, 1969 The coupling device is circular and symmetrical about the axis and comprises, in addition to the plate 15, a coupling hole 19 and an annular tuning stub 20 which penetrates from the plate 15 across the guide 16 through the aperture of a diaphragm portion 21 with the nearest section 14 of the retarded wave structure 14. The outer diameter of the stub 20 and of the coupling hole 19 and the depth of penetration of the said stub through the diaphragm aperture into the structure 14 are chosen in accordance with known principles to obtain transfer of high frequency energy from said structure to the wave guide 16 with minimum reflection and absorption of energy over the intended working band of frequencies of the tube.

The practical requirements as respects coupling out of energy with low reflection and absorption are in many cases very onerous. For example it may be-required to keep losses by absorption down to 2% and losses by refiection down to 1% at all frequencies in a working frequency band 1 g.c./s. wide centered on an operating frequency of 10 g.c./s. Known travelling wavetubes as illustrated by FIGURES 1 to 3 in practice fall considerably short of requirements as onerous as this and often exhibit substantially higher losses at certain frequencies within the working frequency band. The present invention seeks to overcome this defect.

According to this invention a travelling wave amplifier tube of the kind referred to includes between the one end of the retarded wave structure thereof and an associated Waveguide or other high frequency transmission line a coupling device which is of the coupling hole and tuning stub type and the shape of which is not a circular shape symmetrical about the axis.

Experiment has shown that the adoption of the axially symmetrical circular shape for the coupling device of a known tube as exemplified by FIGURES 1 to 3 results in the said device resonating at one or more frequencies within the working band of a high frequency wide band travelling wave tube and this causes considerable absorption of high frequency power at such frequency or frequencies. By departing, in accordance with this invention, from the hitherto employed axially symmetrical circular shape, resonance of the coupling device within the working frequency band is avoided and substantially improved performance is obtained.

The invention may be carried into effect by making the coupling hole of other than simple circular shape. One way of doing this is to provide radial slots in a diaphragm portion through which the tuning stub projects said slots extending radially outwards from the edge of what would otherwise be a simple axially symmetrical circular aperture in said diaphragm portion. Another way is to make the said aperture in the diaphragm portion elliptical in shape instead of circular. Yet another way is to make the aperture rectangular or shaped to the form of a non-regular polygon.

The invention may also be carried into effect by making the tuning stub of other than simple circular shape. One way of doing this is to bifurcate the stub. Another is to make the stub elliptical in shape instead of circular. Yet another way is to make the stub rectangular or shaped to the form of a non-regular polygon.

The two methods of carrying out the invention may be used in conjunction i.e. any of the forms above described for the shape of the aperture in the diaphragm portion may be used in conjunction with any of the forms above described for the shape of the tuning stub.

Various embodiments of the invention are illustrated, so far as is necessary to an understanding thereof in FIGURES 4 to 9 inclusive each of which is a view of the nature of FIGURE 3 and each of which shows a modification which may be made to the known configura- 3 tion shown in FIGURE 3 to bring it into accord with this invention. FIGURES 4 to 9 will, it is thought, he found almost self-explanatory.

In FIGURE 4 radial slots 22 are out in the diaphragm portion 21 separating the guide 16 from the retarded wave structure 14, of which the nearest section 14' is illustrated, these slots extending radially outwards from the edge of what would otherwise be a simple circular aperture in said diaphragm portion. The number and dimensions of the slot are not critical but for a case in which the diameter of the circular portion of the aperture is 0.5 the provision of two slots each wide and /a" long (radially) has been found satisfactory.

In FIGURE 5 the aperture in the diaphragm portion is elliptical and in FIGURE 6 it is rectangular.

FIGURE 7 shows another embodiment in which the tuning stub is bifurcated, being formed with longitudinal cuts or slots 23.

In FIGURE 8 the tuning stub is elliptically shaped and in FIGURE 9 it is rectangular.

We claim:

1. A travelling wave tube including an electron source, an electron collector and a delay line therebetween, coupling me'ans for coupling a cavity of the delay line to an associated waveguide including a member defining a coupling hole and a tuning stub having an end proximate the periphery of said coupling hole, said tuning stub and the periphery of said hole defining a clearance therebetween and about the end of said stub, said stub and coupling hole defining means for reducing resonance occurring at frequencies in the working band width of the travelling wave tube including means providing a non-circular configuration of one of said tuning stub end and said coupling hole periphery hole.

2. A tube as claimed in claim 1 wherein the coupling hole is of other than simple circular shape.

3. A tube as claimed in claim 1 wherein said hole is provided in a diaphragm portion through which the tun- .4 ing stub projects, said hole including radial slots extending radially outwards from the edge of what would otherwise be a simple axially symmetrical circular aperture in said diaphragm portion.

4. A tube as claimed in claim 1 wherein said member defining a coupling hole comprises a diaphragm portion through which the tuning stub projects having an aperture of elliptical shape.

5. A tube as claimed in claim 1 wherein said member defining a coupling hole comprises a diaphragm portion through which the tuning stub projects having an aperture of rectangular shape.

6. A tube as claimed in claim 1 wherein said member defining a coupling hole comprises a diaphragm portion through which the tuning stub projects having an aperture of irregular polygonal shape.

7. A tube as claimed in claim 1 wherein the tuning stub is of other than simple circular shape.

8. A tube as claimed in claim 7 wherein the stub is bifurcated.

9. A tube as claimed in claim 7 wherein the stub is of elliptical shape.

10. A tube as claimed in claim 7 wherein the stub is of rectangular shape.

11. A tube as claimed in claim 7 wherein the stub is of irregular polygonal shape.

References Cited UNITED STATES PATENTS 3,169,206 2/1965 Nelson 3155.39

HERMAN K. SAALBACH, Primary Examiner.

SAXFIELD CHATMON, JR., Assistant Examiner.

US. Cl. X.R. 

